An electrochromic device is a device which is formed by conductive materials and is able to induce a reversible redox reaction through an applied electric field or current, thereby generating color change. Fabrication of an electrochromic device must meet the following characteristics. Colors shown under different voltages must be easily distinguishable, color change is fast and uniform, reversible color change of the device colors over ten thousand times, and the device has high stability. Common electrochromic devices include solid state surface confined thin film electrochromic devices or solution type electrochromic devices.
A solid state surface confined thin film electrochromic device is formed by an upper transparent substrate, a lower transparent substrate, and an electrochromic multi-layered film in between. The electrochromic multi-layered film is similar to a structure of a battery, including at least five coated/plated layers with different functions, such as a transparent conductive layer, an electrochromic layer, an electrolyte layer, an ion storage layer, and a transparent conductive layer. A solution type electrochromic device has a more simple structure, including an upper transparent conductive substrate and a lower transparent conductive substrate. By using epoxy glue, the two substrates are adhered with electrode layers facing each other, and an electrochromic organic solution is disposed in between. An ingredient of the solution includes an oxidation or reduction type small organic molecule electrochromic material, a polymer electrolyte, and a solvent.
Although electrochromic technology has been researched for years, since in a large-area electrochromic device, electrodes are located at peripheries of the device, electric field paths at a planer center and at edges of the electrochromic device have different lengths, impedance values at peripheral regions and at center regions are significantly different. Due to the difference in impedance values, concentric oval-shaped gradients of color change concentrations occur from the edges to the central region, thereby affecting uniformity of color changing effects.
In order to expand an application range of electrochromic technology, a number of studies related to combinations of photoelectric technology and solar cells provide more variable research directions. For example, U.S. Pat. No. 6,369,934B1 discloses a whole organic multi-layer photoelectrochemical device, in which a photosensitive layer and an electrochromic layer are separated at two electrodes with opposite polarity, so as to facilitate assembly of the device. Such a device may be described as having an electrochromic material embedded into a dye-sensitized solar cell. An electrochromic material thereof is WO3, and the device mainly utilizes ruthenium dye. The device includes two transparent conductive substrates and a working electrode layer formed by a photosensitive material, an electrolyte layer, and an counter electrode layer formed by an electrochromic material. However, to apply such a structure to practical applications, many problems need to be overcome such as the long term stability of the photosensitive layer and the possibility of developing devices having larger sizes.
Moreover, U.S. Pat. No. 5,377,037 has disclosed a design of combining a solar cell with an electrochromic device to form one single device, which is basically manufactured by using a stacking method to combine monolithic silicon thin-film solar cells with an inorganic electrochromic device on a first conductive glass substrate, and then oppositely bonding the silicon thin-film solar cells with another transparent conductive glass substrate. Between the substrates, a liquid organic electrolyte or a solid inorganic electrolyte layer is disposed. However, an optical contrast before and after color change obtained through such a technology is relatively low, and thus color changing effect is insignificant. The transparency of the PV-EC device is improved by the incorporation of wide bandgap semitransparent amorphous silicon carbon alloy. However, as the semitransparent PV thin films become very thin, electrical short circuit occurs easily, makes it less attractive to apply the device in smart windows.
US Patent Application Publication No. 2010/0000590 A1 and U.S. Pat. No. 7,855,822 provide several liquid type photovoltaic electrochromic devices, wherein electrodes are uniformly dispersed on a whole surface of a substrate, so that a uniform electric field is formed and an electrochromic solution has a same degree of color change at different regions, thereby preventing iris effects.
Although the above patent applications improve uniformity of color change, problems of leakage easily occur, due to the electrochromic solutions and liquid electrolytes used.